1. Field of the Invention
The present invention relates to the field of semiconductor processing and more particularly to cleaning a semiconductor substrate with cleaning solutions tailored to clean particular surfaces on the semiconductor substrate during processing.
2. Discussion of Related Art
The surfaces of semiconductor substrates may be cleaned at several points during the fabrication of structures on the semiconductor substrates. The cleaning requirements vary for different materials and different structures. The cleaning requirements for surfaces before metallization (front-end-of-the-line) may be different from the cleaning requirements after metallization (back-end-of-the-line). In the past, front-end-of-the-line (FEOL) cleaning was performed using aqueous solutions. In the FEOL, shallow trench isolation structures and gate or memory stacks are typically formed. These structures are fragile due to their increasingly small dimensions and the types of materials used to form the structures. The back-end-of-the-line (BEOL) may also have fragile submicron structures such as dual damascene etched openings in low-k dielectric materials or polysilicon interconnect lines. In particular, structures having a dimension of 0.15 μm or less, require special cleaning considerations. Polysilicon is a fragile material, particularly when it is part of a gate stack formed on a thin silicon oxide layer. In the past, fragile structures in the FEOL were cleaned using aqueous solutions such as SCl without any additional agitation such as acoustic energy. The use of acoustic energy was found to cause significant damage in combination with aqueous solutions. For example, the use of acoustic energy in combination with aqueous solutions may cause the loss of entire portions 110 of polysilicon lines 100 as illustrated in FIG. 1a. But, using only an aqueous solution does not adequately clean the surfaces of submicron structures having a dimension of 0.15 μm or less and therefore may cause the loss of critical dimension control and ultimately cause lower yields.
In the past, BEOL substrates containing copper were cleaned with basic amine containing cleaning solutions. These cleaning solutions typically contained a solvent, an amine to remove organic residues, a fluoride ion to remove oxide and copper residues, and an inhibitor to prevent copper etching by the amine. Solvent based cleaning solutions were used because they were effective at removing large amounts of photoresist residues present after the etching steps. As technology has advanced, the need to remove large amounts of photoresist residues has been reduced. Photoresists are now typically removed by plasma ashing and very few photoresist residues are left behind. Therefore, the use of the basic, solvent-based and amine containing cleaning solutions are no longer necessary but are still typically used. Because the residues remaining after photoresist removal are now mainly inorganic rather than organic the use of amines is not required. Also, because the amine based cleaning solution was developed for removing mainly organic residues, it is not as effective at cleaning mostly inorganic residues. Additionally, the amines left over after cleaning may cause photoresist poisoning in later steps. Additionally, the solvents and the chemical additives used in amine-based cleaning solutions are expensive and difficult to dispose of in an environmentally friendly manner. Aqueous solutions such as hydrofluoric acid in water and oxalic acid in water have been used to clean dual damascene structures, but due to the surface tension an aqueous solution 125 may distort or pull down the interlayer dielectric 115, as illustrated at 120 in FIG. 1b. Additionally, the aqueous HF cleaning solution and the aqueous oxalic acid cleaning solution do not remove residues to the extent necessary to provide the desired critical dimension control and improved yield.
BEOL copper processing often uses a dual damascene process including many steps at which the wafer is cleaned, both front and back. In the past, to clean both sides of the wafer in a single wafer cleaning tool would require the removal and flipping of the wafer. Also, both sides of the wafer have been cleaned in a single wafer cleaning tool by spraying cleaning solutions on both the top 130 and the bottom 135 of a wafer 140, as illustrated in FIG. 1c. Cleaning both sides of the wafer using the dual spray method has the drawback of the splashing 145 caused by spraying a cleaning solution onto the bottom of a wafer. The splashing will result in the mixing of the cleaning solution applied to the top 130 and the bottom 135 of the wafer 140, therefore making it difficult to use two different cleaning solutions on the top and the bottom of the wafer, particularly where the mixing of the two cleaning solutions would result in the degradation of one or both of the cleaning solutions.